fix: Add buffer polyfill with base64url support

[felipecsl]

Fixes: FF-3645

Motivation and Context

Add buffer fork polyfill with support for base64url encoding.
Code is here https://github.com/Eppo-exp/buffer/tree/add-base64url-support
Source: feross/buffer#314 (once this is merged, we can revert back to the main buffer repo)

Description

Make sure we polyfill buffer for browser environment and it has support for base64url.

Thanks @maya-the-mango for reporting and helping with debugging

How has this been tested?

• Started a new next.js repo and added a dependency on "@eppo/js-client-sdk": "^3.8.0",
• Also added resolution to this fork:

  "resolutions": {
    "@eppo/js-client-sdk-common": "git+https://github.com/eppo-exp/js-sdk-common#felipecsl--buffer-polyfill"
  }

called init and verified no errors thrown

[maya-the-mango]

Thanks @felipecsl for quick reaction. I confirm that it solves the buffer error that I previously encountered!